1. Field of the Invention
The present invention relates to a heat-sensitive recording material, and in particular to a heat-sensitive recording material capable of forming high-density color images with little background fogging and having good image preservability.
2. Description of the Related Art
Heat-sensitive recording materials are widely used in the art because they are relatively inexpensive and capable of being processed in compact recording appliances not requiring specific maintenance. In order to increase the density of the color images to be formed on such heat-sensitive recording materials and to improve the image preservability of the materials, various studies are now being made relating to electron-donating leuco-dyes and electron-receiving compounds and to the layer constitution of heat-sensitive recording materials.
2,2-bis(4-hydroxyphenyl)propane (i.e., bisphenol A, referred to as xe2x80x9cBPAxe2x80x9d) has been widely used as an electron-receiving compound with respect to electron-donating leuco-dyes used in such heat-sensitive recording materials. However, a heat-sensitive recording material having satisfactory sensitivity, ability to prevent background fogging, and good image preservability has not been obtained.
Japanese Patent Application Publication (JP-B) No. 2-25354 discloses a heat-sensitive recording paper in which N-(4-hydroxyphenyl)-p-toluenesulfonamide is used as an electron-receiving compound. However, with the advancement in recent years of high-speed thermal head printers, even such heat-sensitive recording papers are unsatisfactory in terms of sensitivity, ability to prevent background fogging, and also in terms of thermal head matching properties such as adhesion of contaminants to the thermal heads and abrading the thermal heads.
In addition, heat-sensitive recording materials generally have weak surface strength and inferior printability which gives rise to problems such as peeling of the paper during offset printing. In order to avoid such problems, it is possible to increase the amount of adhesive included in the heat-sensitive color-forming layer. However, increasing the amount of adhesive is problematic in that the color density of the heat-sensitive recording material is lowered. Thus, in order to solve these problems, there has been a strong demand for heat-sensitive recording materials that can ensure high color density and have good printability.
Further, inkjet printers have become widespread in offices as means for outputting from personal computers. Respective recording surfaces of the inkjet recording materials and heat-sensitive recording materials are often put together. However, conventional heat-sensitive recording materials are not satisfactorily resistant to ink for inkjet printers. Therefore, when the recording surface of the heat-sensitive recording material contacts the recording surface of the inkjet recording material, there has been the problem of fogging in the background area of the heat-sensitive recording material and density of the image area being lowered.
Moreover, due to a heightening awareness of the environment in recent years, there has been a demand for heat-sensitive recording materials that utilize supports in which waste pulp (so-called recycled paper) forms the main constituent. However, it has not always been the case that satisfactory heat-sensitive recording materials have been obtained, because background fogging and image preservability become worse as a result of using recycled paper as the support. In particular, when BPA is used as a developer for the recycled paper, background fogging and image preservability deteriorate.
Japanese Patent Application Laid-Open (JP-A) No. 3-140287 discloses a heat-sensitive recording material with which recording sensitivity is improved without attendant background contamination, and that can be applied to ultra-high speed printers as a result. This is due to the use of a recycled paper, wherein a measured value of a base paper by a regular reflection-type surface smoothness sensor under a pressure of 20 kg/cm2 is no less than 8%. The heat-sensitive recording material disclosed in JP-A No. 3-140287 uses a developer comprising a phenol (such as bisphenol), a sulfone and a hydroxybenzoic acid. However, the image preservability of the heat-sensitive recording material is insufficient.
JP-A No. 4-21486 discloses obtaining a heat-sensitive recording material that has excellent recoloring potential (coloring after preservation), even when recycled paper is used as the support, by using as the developer bis(4-hydroxyphenyl)acetate-n-butyl, 4-hydroxy-4-isopropoxydiphenyl sulfone, 4,4xe2x80x2-thiobis(3-methyl-6-tert-butylphenol) or N,Nxe2x80x2-diphenylthiourea. However, resistance to background fogging and image preservability of the heat-sensitive recording material disclosed in JP-A No. 4-21486 are still insufficient.
It is a first object of the present invention to provide a heat-sensitive recording material that ensures increased color density, little background fogging and good image preservability.
It is a second object of the present invention to provide a heat-sensitive recording material having good chemical resistance.
It is a third object of the present invention to provide a heat-sensitive recording material that well matches thermal heads, without leaving contaminants on the thermal heads and without abrading the thermal heads.
It is a fourth object of the present invention to provide a heat-sensitive recording material having good printability.
It is a fifth object of the present invention to provide a heat-sensitive recording material resistant to ink used in inkjet recording systems.
It is a sixth object of the present invention to provide a heat-sensitive recording material which comprises recycled paper of essentially waste pulp as the support and which therefore has the advantage of reducing the burden upon the environment.
It is a seventh object of the present invention to provide a heat-sensitive recording material having sticking resistance.
Specifically, the present invention provides a heat-sensitive recording material comprising support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound and includes a sensitizer of 2-benzyloxynaphthalene.
With the heat-sensitive recording material comprising the above structure, color density is raised, there is little background fogging, and preservability of image areas and chemical resistance is excellent in comparison with conventional heat-sensitive recording materials.
The present invention also provides a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound and includes an inorganic pigment of calcite-type precipitated calcium carbonate light and/or aluminium hydroxide.
With the heat-sensitive recording material comprising the above structure, color density is raised, there is little background, preservability of image areas is excellent, and compatibility with thermal heads is excellent (contaminants do not adhere to the thermal heads and the thermal heads are not abraded) in comparison with conventional heat-sensitive recording materials.
The present invention also provides a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound and includes an adhesive of polyvinyl alcohol having a degree of saponification of 85 to 99 mol % and a degree of polymerization of 200 to 2000.
With the heat-sensitive recording material comprising the above structure, sensitivity is increased, there is little background fogging, and preservability of image areas and offset printability are improved in comparison with conventional heat-sensitive recording materials.
The present invention also provides a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, with a protective layer being disposed on the heat-sensitive color-forming layer, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound and the protective layer includes an inorganic pigment and a water-soluble polymer.
With the heat-sensitive recording material comprising the above structure, inkjet ink resistance and sensitivity are improved, there is little background fogging, and color image preservability is excellent.
The present invention also provides a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound and the support comprises essentially waste pulp.
Although recycled paper is used as the support, by using a specific developer, the heat-sensitive recording material of the present invention has well-balanced properties of high sensitivity, little background fogging and excellent image preservability.
The present invention also provides a heat-sensitive recording material comprising a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, wherein the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound, and includes, as a sensitizer, 2-benzyloxynaphthalene and methylolstearic acid amide, with a ratio (x/y) of the 2-benzyloxynaphthalene (x) to the methylolstearic acid amide (y) being 95/5 to 40/60, and includes, as an image stabilizer, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane.
With the heat-sensitive recording material comprising the above structure, color density is raised, there is little background fogging, and preservability of image areas, chemical resistance and sticking resistance are improved in comparison with conventional heat-sensitive recording materials.
A heat-sensitive recording material of the present invention will be described below. The heat-sensitive recording material comprises a support having disposed thereon a heat-sensitive color-forming layer that includes an electron-donating leuco-dye and an electron-receiving compound, with the heat-sensitive color-forming layer including N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound. Conventional, well known supports can be used as the support in the present invention. Specifically, the support may comprise paper, such as woodfree paper, a paper to which a resin or pigment has been coated, resin-laminated paper, base paper having an undercoat layer (especially, woodfree paper having an undercoat layer), a synthetic paper, or a plastic film.
In order to improve thermal head matching property, base paper having an undercoat layer is preferable. A base paper to which an undercoat layer that includes an oil-absorbing pigment has been disposed with a blade coater is even more preferable. In this case, it is preferable that the Stoeckigt sizing degree of the support is at least 5 seconds.
As the above support, a smooth support having a surface smoothness of at least 300 seconds, as stipulated by JIS-P8119, is preferable from the standpoint of good dot reproducibility. More preferably, the degree of surface smoothness of the support falls between 300 seconds and 500 seconds.
A support including waste pulp as a main constituent, i.e., a support in which waste pulp occupies 50 wt. % of the support, can also be used.
Waste pulp is generally made from a combination of three steps:
(1) disaggregation, in which waste paper is processed by a pulper with chemicals and mechanical force to be disaggregated into fibers, whereby printed ink in the waste paper is separated from the fibers;
(2) removal of impurities, in which impurities (plastic, etc.) and other contaminants that were in the waste paper are removed; and
(3) deinking, in which the ink separated from the fibers is removed from the system by floatation or washing.
If desired, the fibers may be bleached while they are deinked or in another step.
Using 100 wt. % of the waste pulp thus obtained, or using a mixture of the waste pulp combined less than 50 wt. % of virgin pulp, the support for the heat-sensitive recording material is formed according to ordinary processes.
From the standpoint of good dot reproducibility, a smooth support having a surface smoothness of at least 100 seconds, and preferably 150 seconds, as stipulated by JIS-P8119, is preferable as the support in which waste pulp forms the main constituent.
Moreover, the support used in the present invention may have an undercoat layer. The undercoat layer preferably has pigment as a main component. All general inorganic and organic pigments may be used. However, the pigment is preferably one having a degree of oil absorption of at least 40 ml/100 g (cc/100 g), as stipulated by JIS-K5101. Specific examples include calcium carbonate, magnesium carbonate, barium sulfate, aluminium oxide, aluminium hydroxide, kaolin, calcined kaolin, amorphous silica, calcined diatomaceous earth, aluminium silicate, magnesium aluminosilicate, and urea-formalin resin powder. Among these, calcined kaolin having a degree of oil absorption of 70 ml/100 g to 80 ml/100 g is preferred.
When these pigments are coated on the support, the amount of the pigment is preferably at least 2 g/m2, more preferably at least 4 g/m2, and even more preferably 7 g/m2 to 12 g/m2.
Examples of the binder used in the undercoat layer include water-soluble polymers and aqueous binders. These may be used singly, or in combination of two or more different.
Examples of the water-soluble polymer include starch, polyvinyl alcohol, polyacrylamide, carboxymethyl cellulose, methyl cellulose and casein.
The aqueous binder generally comprises synthetic rubber latex or synthetic resin emulsion. Examples thereof include styrene-butadiene rubber latex, acrylonitrile-butadiene rubber latex, methyl acrylate-butadiene rubber latex and vinyl acetate emulsion.
The amount of the binder used is determined in view of film strength of the coating layer and heat sensitivity of the heat-sensitivity color-forming layer. The amount of the binder with respect to the pigment added to the undercoat layer is 3 to 100 wt. %, preferably 5 to 50 wt. %, and more preferably 8 to 15 wt. %. The undercoat layer may also include, for example, wax, color erasure inhibitors and surfactants.
Any known coating method can be employed to apply the undercoat layer. Specifically, methods can be used that use air-knife coaters, roll coaters, blade coaters, gravure coaters, and curtain coaters. Among these, methods that use blade caters are preferable. Further, a smoothing treatment such as calendering may also be administered to the undercoat layer as needed.
Methods that use blade coaters are not limited to methods that used bevel blade coaters or vented blade coaters, and include methods that use rod blade coaters and bill blade coaters. These methods are also not limited to off-machine coaters. The undercoat layer may be applied by an on-machine coater disposed in a papermaking machine. For enhancing flowability when the undercoat layer is applied by the blade coater, and to obtain excellent smoothness and planar shape, carboxymethyl cellulose having a degree of etherification of 0.6 to 0.8 and a weight-average molecular weight of 20000 to 200000 may be added to the coating liquid at 1% to 5% by weight, preferably 1% to 3% by weight, with respect to the pigment.
The heat-sensitive color-forming layer formed on the support includes at least an electron-donating leuco-dye and an electron-receiving compound, and may optionally include sensitizers, inorganic pigments, image stabilizers, adhesives, UV absorbents and crosslinking agents.
The electron-donating leuco-dye is preferably at least one selected from 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-(N-ethyl-N-isoamylamino)fluoran and 2-anilino-3-methyl-6-(N-ethyl-N-propylamino)fluoran. These may be used singly, or two or more may be used in combination.
By using at least one selected from 2-anilino-3-methyl-6-diethylaminofluoran, 2-anilino-3-methyl-6-(N-ethyl-N-isoamylamino)fluoran and 2-anilino-3-methyl-6-(N-ethyl-N-propylamino)fluoran as the electron-donating leuco-dye, color density, preservability of image areas and chemical resistance can be further improved.
In addition to the above, for example, 3-di(n-butylamino)-6-methyl-7-anilinofluoran, 2-anilino-3-methyl-6-N-ethyl-N-sec-butylaminofluoran, 3-di(n-pentylamino)-6-methyl-7-anilinofluoran, 3-(N-isoamyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-(N-n-hexyl-N-ethylamino)-6-methyl-7-anilinofluoran, 3-[N-(3-ethoxypropyl)-N-ethylamino]-6-methyl-7-anilinofluoran, 3-di(n-butylamino)-7-(2-chloroanilino)fluoran, 3-diethylamino-7-(2-chloroanilino)fluoran, 3-diethylamino-6-methyl-7-anilinofluoran, and 3-(N-cyclohexyl-N-methylamino)-6-methyl-7-anilinofluoran may also be used as the electron-donating leuco-dye.
Other than 2-anilino-3-methyl-6-diethylaminofluoran, and the like, from the standpoint of background fogging of non-image areas, 3-di(n-butylamino)-6-methyl-7-anilinofluoran, 2-anilino-3-methyl-6-N-ethyl-N-sec-butylaminofluoran and 3-diethylamino-6-methyl-7-anilinofluoran are preferable as the electron-donating leuco-dye.
The coating amount of the electron-donating leuco-dye is preferably 0.1 to 1.0 g/m2, and more preferably 0.2 and 0.5 g/m2 in view of color density and background fogging density.
The heat-sensitive recording material of the present invention includes N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound. By including N-(4-hydroxyphenyl)-p-toluenesulfonamide as the electron-receiving compound, color density is raised, background fogging is reduced and preservability of image areas is improved.
The amount of the electron-receiving compound is preferably 50 to 400% by weight, more preferably 10 to 300% by weight, and even more preferably 100 to 300% by weight with respect to the electron-donating leuco-dye.
Well known electron-receiving compounds other than N-(4-hydroxyphenyl)-p-toluenesulfonamide may also be used together therewith as the electron-receiving compound in the present invention as long as the effects of the present invention are not compromised.
Although any electron-receiving compound may be suitably selected for use in the invention, phenolic compounds or salicylic acid derivatives and their polyvalent metal salts are preferable from the standpoint of preventing background fogging.
Examples of the phenolic compounds include 2,2xe2x80x2-bis(4-hydroxyphenol)propane (bisphenol A), 4-t-butylphenol, 4-phenylphenol, 4-hydroxy-diphenoxide, 1,1xe2x80x2-bis(4-hydroxyphenyl)cyclohexane, 1,1xe2x80x2-bis(3-chloro-4-hydroxyphenyl)cyclohexane, 1,1xe2x80x2-bis(3-chloro-4-hydroxyphenyl)-2-ethylbutane, 4,4xe2x80x2-sec-isooctylidene-diphenol, 4,4xe2x80x2-sec-butylidene-diphenol, 4-tert-octylphenol, 4-p-methylphenylphenol, 4,4xe2x80x2-methylcyclohexylidene-phenol, 4,4xe2x80x2-isopentylidene-phenol, 4-hydroxy-4-isopropyloxydiphenyl sulfone, benzyl p-hydroxybenzoate, 4,4xe2x80x2-dihydroxydiphenyl sulfone, and 2,4xe2x80x2-dihydroxydiphenyl sulfone.
Examples of the salicylic acid derivatives and their polyvalent metal salts include 4-pentadecylsalicylic acid, 3,5-di(xcex1-methylbenzyl)salicylic acid, 3,5-di(tert-octyl)salicylic acid, 5-octadecylsalicylic acid, 5-xcex1-(p-xcex1-methylbenzylphenyl)ethylsalicylic acid, 3-xcex1-methylbenzyl-5-tert-octylsalicylic acid, 5-tetradecylsalicylic acid, 4-hexyloxysalicylic acid, 4-cyclohexyloxysalicylic acid, 4-decyloxysalicylic acid, 4-docecyloxysalicylic acid, 4-pentadecyloxysalicylic acid, 4-octadecyloxysalicylic acid, and their salts with zinc, aluminium, calcium, copper or lead.
When N-(4-hydroxyphenyl)-p-toluenesulfonamide is used together with any of the above-mentioned known electron-donating compounds, the amount of N-(4-hydroxyphenyl)-p-toluenesulfonamide is preferably at least 50% by weight, more preferably at least 70% by weight of the total of the electron-donating compounds.
In preparing the coating liquid for the heat-sensitive color-forming layer, the volume-average particle diameter of the particles of the electron-donating compound is preferably at most 1.0 xcexcm, more preferably 0.4 to 0.7 xcexcm, and even more preferably 0.5 to 0.7 xcexcm. If the volume-average particle diameter of the particles exceeds 1.0 xcexcm, sometimes the heat sensitivity is lowered. The volume-average particle diameter can be readily measured with a laser-diffractometric particle size distribution analyzer (e.g., LA500, manufactured by Horiba).
The heat-sensitive color-forming layer in the heat-sensitive recording material of the present invention may include a sensitizer. Examples thereof include 2-benzyloxynaphthalene, aliphatic monoamides such as stearic acid amides (especially, methylolstearic acid amide) and palmitic acid amides, as well as stearylurea, p-benzylbiphenyl, di(2-methylphenoxy)ethane, di(2-methoxyphenoxy)ethane, xcex2-naphthol (p-methylbenzyl)ether, xcex1-naphthyl benzyl ether, 1,4-butanediol p-methylphenyl ether, 1,4-butanediol p-isopropylphenyl ether, 1,4-butanediol p-tert-octylphenyl ether, 1-phenoxy-2-(4-ethylphenoxy)ethane, 1-phenoxy-2-(chlorophenoxy)ethane, 1,4-butanediol phenyl ether, diethylene glycol bis(4-methoxyphenyl) ether, m-terphenyl, methyl oxalate benzyl ether, 1,2-diphenoxymethylbenzene, and 1,2-bis(3-methylphenoxy)ethane, 1,4-bis(phenoxymethyl)benzene. By including such sensitizers, the sensitivity of the recording material is significantly increased. Among the above sensitizers, 2-benzyloxynaphthalene and aliphatic monoamides are preferable, and 2-benzyloxynaphthalene is particularly preferable. When 2-benzyloxynaphthalene and methylolstearic acid amide are included, it is possible to greatly improve sensitivity while suppressing generation of background fogging.
The amount of the sensitizer added is preferably 75 to 200 parts by weight, and more preferably 100 to 150 parts by weight, relative to 100 parts by weight of N-(4-hydroxyphenyl)-p-toluenesulfonamide, which is the electron-receiving compound. When the amount of the sensitizer is within the range of 75 to 200 parts by weight, the effects of improved sensitivity become more pronounced and image preservability is also good.
When 2-benzyloxynaphthalene and methylolstearic acid amide are included, the ratio (x/y) of 2-benzyloxynaphthalene (x) to methylolstearic acid amide (y) by weight is preferably 95/5 to 40/60. When the ratio is smaller than 95/5 or is greater than 40/60, in either case sensitivity becomes low. The ratio by weight is more preferably 90/10 to 50/50, and even more preferably 85/15 to 70/30.
The heat-sensitive color-forming layer in the heat-sensitive recording material of the present invention may include as an inorganic pigment calcium carbonate (e.g., calcite-type, cubic system), aluminium hydroxide, barium sulfate, magnesium carbonate, magnesium oxide, lithopone, agalmatolite, kaolin, calcined kaolin and amorphous silica.
Among these, basic pigments such as calcium carbonate, aluminium hydroxide, basic magnesium carbonate and magnesium oxide are preferably used, from the standpoint of obtaining a heat-sensitive recording material in which there is little background fogging. When calcite-type precipitated calcium carbonate light and/or aluminium hydroxide are included, there is little background fogging, little abrading of the thermal head, little contaminants adhere to the thermal head them and sticking resistance is improved.
Precipitated calcium carbonate light is preferable. The crystal form of the precipitated calcium carbonate light is generally, for example, calcite (burr-like configuration), aragonite or vaterite. However, calcite-type precipitated calcium carbonate light is preferable in view of absorbability, hardness and color density when recorded by a thermal head. Calcite-type precipitated calcium carbonate light whose particle shapes are spindle-like or scalenohedral is even more preferable.
Well known methods can be used to manufacture the calcite-type precipitated calcium carbonate light.
The amount of the inorganic pigment to be in the heat-sensitive color-forming layer is preferably 50 to 250 parts by weight, more preferably 70 to 170 parts by weight, and even more preferably 90 to 140 parts by weight, relative to 100 parts by weight of the electron-receiving compound in the layer, in view of color density and reducing adhesion of contaminants to the thermal head. The amount of the pigment is preferably 50 to 1000% by weight, and more preferably 100 to 500% by weight, with respect to the electron-donating leuco-dye.
With respect to the particle diameter of the inorganic pigment, the volume-average particle diameter is preferably 0.6 to 2.5 xcexcm, more preferably 0.8 to 2.0 xcexcm, and even more preferably 1.0 to 1.6 xcexcm, in view of color density and reducing adhesion of contaminants to the thermal head. Moreover, a burr-like (calcite-type) calcium carbonate having a particle diameter of 1 to 3 xcexcm can be preferably used. Further, kaolin having a particle diameter 1 to 3 xcexcm can also be preferably used. The mean particle diameter of the other pigments such as aluminium hydroxide is preferably within the range of 0.3 to 1.5 xcexcm, and more preferably 0.5 to 0.9 xcexcm.
When calcite-type precipitated calcium carbonate light (x) and aluminium hydroxide (y) are used together, the weight ratio (x/y) is preferably 80/20 to 20/80, and more preferably 60/40 to 40/60.
When calcite-type precipitated calcium carbonate light and aluminium hydroxide are combined with other inorganic pigments, it is preferable that the ratio (v/w) by weight of the total amount (v) of calcite-type precipitated calcium carbonate light and aluminium hydroxide to the total amount (w) of the other inorganic pigments is 100/0 to 60/40, and more preferably 100/0 to 80/20.
It is also preferable for basic magnesium carbonate and magnesium oxide to be combined with other pigments and used, in view of background fogging. In this case, the amount of basic magnesium carbonate and magnesium oxide is preferably 3 to 50% by weight, and more preferably 5 to 30% by weight, of the total pigments.
The heat-sensitive color-forming layer may include as an adhesive polyvinyl alcohol (hereinafter, sometimes referred to as xe2x80x9cPVAxe2x80x9d) having a degree of saponification of 85 to 99 mol % and a degree of polymerization of 200 to 2000. By including such PVA as an adhesive in the heat-sensitive color-forming layer, adhesion between the heat-sensitive color-forming layer and the support is increased and problems such as paper peeling that arise during offset printing are prevented, whereby printability is improved.
PVA that has a degree of saponification of 85 to 99 mol % can be used in the present invention. When the degree of saponification of the PVA is less than 85 mol %, resistance to moisture used in offset printing is insufficient and it becomes easy for paper peeling to occur. Alternatively, when the amount of PVA added is increased in order to prevent paper peeling, color density is lowered. Further, when the degree of saponification exceeds 99 mol %, it becomes easy for undissolved matter to arise at the time the coating solution is prepared, which leads to defects and is therefore undesirable.
PVA that has a degree of polymerization within the range of 200 to 2000 can also be used in the present invention. When the degree of polymerization of the PVA is less than 200, it becomes easy for paper peeling to occur at the time of offset printing. When the amount of PVA added is increased in order to prevent paper peeling, color density is lowered. When the degree of polymerization exceeds 2000, it becomes difficult for the PVA to be dissolved in the solvent and the viscosity of the coating solution increases, whereby it becomes difficult to prepare and coat the coating solution. The degree of polymerization referred to herein indicates a mean degree of polymerization measured by methods disclosed in JIS-K6726 (1994).
The PVA content in the heat-sensitive color-forming layer is preferably 30 to 300 parts by weight relative to 100 parts by weight of the electron-donating leuco-dye, in view of color density and offset printability of the recording material (no paper peeling, etc.). The PVA content is more preferably 70 and 200 parts by weight, and even more preferably 100 to 170 parts by weight. The PVA in the heat-sensitive color-forming layer of the present invention acts not only as an adhesive for increasing adhesion between the support and the heat-sensitive color-forming layer, but also as a dispersant and a binder.
Any PVA that having a degree of saponification of 85 to 99 mol % and a degree of polymerization of 200 and 2000 can be suitable used. However, at least one selected from sulfo-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol and acetoacetyl-modified polyvinyl alcohol is preferable in view of color density in recording by a thermal head.
The sulfo-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol and acetoacetyl-modified polyvinyl alcohol may be used either singly or in combination, or may be further combined with another PVA. When combined with another PVA, the amount of sulfo-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol and/or acetoacetyl-modified polyvinyl alcohol is preferably at least 10% by weight, and more preferably at least 20% by weight, with respect to the total amount of the PVA.
The sulfo-modified polyvinyl alcohol can be prepared by several different methods, such as: a method in which an olefinsulfonic acid or its salt, such as ethylenesulfonic acid, allylsulfonic acid or methallylsulfonic acid, is polymerized with a vinyl ester such as vinyl acetate in an alcohol or in a mixed solvent of alcohol/water, followed by saponifying the resultant polymer; a method in which a sulfonamide sodium salt is copolymerized with vinyl ester such as vinyl acetate, followed by saponifying the resultant copolymer; a method in which a PVA is treated with bromine or iodine, followed by heating the treated PVA in an aqueous solution of acidic sodium sulfite; a method in which a PVA is heated in an aqueous solution of concentrated sulfuric acid; and a method in which a PVA is acetalyzed with an aldehyde compound having a sulfonic acid group.
The diacetone-modified polyvinyl alcohol is a partially or completely saponified product of a copolymer of a vinyl ester and a monomer having a diacetone group. The diacetone-modified polyvinyl alcohol is prepared, for example, by saponifying a resin obtained by copolymerizing a vinyl ester and a monomer having a diacetone group.
In the diacetone-modified polyvinyl alcohol, there are no particular limitations on the amount of the monomer having the diacetone group (repetitive unit structure).
The acetoacetyl-modified polyvinyl alcohol is generally prepared by adding a liquid or gaseous diketene to a solution, dispersion or powder of polyvinyl alcohol resin and reacting them. The degree of acetylation of the acetoacetyl-modified polyvinyl alcohol can be suitably determined in accordance with the desired quality of the heat-sensitive recording material.
The heat-sensitive color-forming layer may also include image stabilizers and UV absorbents.
For the image stabilizer, phenolic compounds, especially hindered phenol compounds are effective. Example thereof include 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane, 1,1,3-tris(2-ethyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane, 1,1,3-tris(3,5-di-tert-butyl-4-hydroxyphenyl)butane, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)propane, 2,2xe2x80x2-methylene-bis(6-tert-butyl-4-methylphenol), 2,2xe2x80x2-methylene-bis(6-tert-butyl-4-ethylphenol), 4,4xe2x80x2-butylidene-bis(6-tert-butyl-3-methylphenol), and 4,4xe2x80x2-thio-bis(3-methyl-6-tert-butylphenol). Among these, 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane is preferable. A combination of 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane is also preferable. When the heat-sensitive color-forming layer includes N-(4-hydroxyphenyl)-p-toluenesulfonamide and 2-benzyloxynaphthalene, background fogging is reduced by the interaction between the image stabilizer and N-(4-hydroxyphenyl)-p-toluenesulfonamide and 2-benzyloxynaphthalene, and preservability of image areas is further improved.
The amount of the image stabilizer in the heat-sensitive color-forming layer is preferably 10 is 100 parts by weight, more preferably 20 to 60 parts by weight, and even more preferably 30 to 60 parts by weight, with respect to 100 parts by weight of the electron-donating leuco dye in the layer, from the standpoint of effectively reducing background fogging and improving image preservability.
When 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane (xcex1) and 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane (xcex2) are combined, the ratio (xcex1/xcex2) by weight is preferably 20/80 to 80/20, and more preferably 40/60 to 60/40.
When 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane is combined with another image stabilizer, the amount of 1,1,3-tris(2-methyl-4-hydroxy-5-tert-butylphenyl)butane and/or 1,1,3-tris(2-methyl-4-hydroxy-5-cyclohexylphenyl)butane is preferably at least 50% by weight, and more preferably at least 70% by weight, of the total amount of the image stabilizer.
Examples of the UV absorbents include the following UV absorbents. 
The heat-sensitive color-forming layer may also include a crosslinking agent to crosslink the PVA that serves as the adhesive. By the heat-sensitive color-forming layer including such a crosslinking agent, moisture resistance of the heat-sensitive recording material is improved.
As long as the crosslinking agent is capable of crosslinking the PVA, any crosslinking agent can be suitably used. However, aldehyde compounds such as glyoxal, and dihydrazide compounds such as adipic acid dihydrazide, are particularly preferable.
The amount of the crosslinking agent is preferably 1 to 50 parts by weight, and more preferably 3 to 20 parts by weight, with respect to 100 parts by weight of the polyvinyl alcohol to be crosslinked by the crosslinking agent and included in the heat-sensitive color-forming layer. It is preferable that the amount of the crosslinking agent is within the range of 1 to 50 parts by weight with respect to the PVA, in view of resistance to moisture.
In the present invention, the electron-donating leuco-dye and the electron-receiving compound can be dispersed in the adhesive or in a water-soluble binder. The water-soluble binder is preferably a compound in which at least 5% by weight is dissolved in water at 25xc2x0 C.
Examples of the water-soluble binder include polyvinyl alcohol, methyl cellulose, carboxymethyl cellulose, starches (including modified starches), gelatin, arabic gum, casein, and saponified copolymers of styrene and maleic anhydride.
The binder may be used not only at the time of dispersion, but also for improving the film strength of the heat-sensitive color-forming layer. To this end, the water-soluble binder may be combined with a synthetic polymer latex binder such as styrene-butadiene copolymer, vinyl acetate copolymer, acrylonitrile-butadiene copolymer, methyl acrylate-butadiene copolymer or polyvinylidene chloride.
The electron-donating leuco-dye and electron-receiving compound may be dispersed together or separately by using a stirring grinder such as a ball mill, an attritor or a sand mill, to then be prepared as the coating liquid. The coating liquid may include metal soaps, waxes, surfactantes, antistatic agents, defoaming agents and fluorescent dyes as needed.
Metal salts of higher fatty acids, such as zinc stearate, calcium stearate and aluminium stearate, can be used as the metal soap.
Paraffin wax, microcrystalline wax, carnauba wax, methylolstearic acid amide, polyethylene wax, polystyrene wax and fatty acid amide-type wax can be used for the wax, either singly or in combination. For the surfactant, alkali metal salts and ammonium salts of alkylbenzenesulfonates, alkali metal salts of sulfosuccinic acids, and surfactants including fluorine can be used.
After these materials are mixed, they are applied to the support. There are no particular limitations on the method for coating. For example, the mixture can be applied with air-knife coaters, roll coaters, blade coaters or curtain coaters, dried, and then leveled with a calender. However, in the present invention, a method that utilizes a curtain coater is particularly preferable, because the heat-sensitive color-forming layer can be coated uniformly, and because sensitivity and image preservability are effectively improved.
There are no particular limitations on the amount of the heat-sensitive color-forming layer. Ordinarily, an amount in which the dry weight thereof is 2 and 7 g/m2 is preferable.
If desired, a protective layer may be provided on the heat-sensitive color-forming layer. The protective layer may include organic fine powders, inorganic fine powders (inorganic pigments), binders, surfactants, and hot-melting substances. Examples of the inorganic pigment include calcium carbonate, silica, zinc oxide, titanium oxide, aluminium oxide, titanium dioxide, silicon dioxide, aluminium hydroxide, zinc hydroxide, barium sulfate, zinc sulfate, kaolin, clay, calcined clay, talc, colloidal silica, surface processed calcium and silica. For the organic fine powder, urea-formalin resin, copolymers of styrene and methacrylic acid, and polystyrene can be used.
Preferable examples of the inorganic pigment include aluminium hydroxide and kaolin. Aluminium hydroxide having a mean particle diameter of 0.5 to 0.9 xcexcm is even more preferable, in view of color density when recording by a thermal head.
The amount of the inorganic pigment added is preferably 10 to 90% by weight, and more preferably 30 to 70% by weight, with respect to the solid content of the coating liquid for the protective layer.
The proportion at which the inorganic pigment and the water-soluble polymer are mixed varies depending on the type of inorganic pigment and the type of water-soluble polymer. However, the amount of the water-soluble polymer is preferably 50 to 400% by weight, and more preferably 100 to 250% by weight, with respect to the inorganic pigment.
The sum of the inorganic pigment and the water-soluble polymer binder in the protective layer is at least 50% by weight of the layer.
Examples of the binder in the protective layer include water-soluble polymers such as polyvinyl alcohol, modified polyvinyl alcohol (e.g., carboxy-modified polyvinyl alcohol, silicon-modified polyvinyl alcohol, acetoacetyl-modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, amide-modified polyvinyl alcohol, sulfo-modified polyvinyl alcohol), vinyl acetate-acrylamide copolymer, starch, oxidized starch, modified starch (e.g., urea phosphate-modified starch), methyl cellulose, carboxymethyl cellulose, hydroxymethyl cellulose, gelatins, arabic gum, casein, copolymers of styrene and maleic anhydride, alkyl esters of copolymers of styrene and maleic anhydride, hydrolyzed coploymers of styrene and maleic acid, polymers having carboxyl groups such as copolymers of styrene and acrylic acid, polyacrylamide derivatives, and polyvinyl pyrrolidone, as well as latexes such as styrene-butadiene rubber latex, acrylonitrile-butadiene rubber latex, methyl acrylate-butadiene rubber latex, and polyvinyl acetate emulsion. Among these, water-soluble polymers are preferable.
Among water-soluble polymers, polyvinyl alcohol, modified polyvinyl alcohol, oxidized starch, and urea phosphate-modified starch are preferable. It is even more preferable to mix polyvinyl alcohol and/or modified polyvinyl alcohol with oxidized starch and/or urea phosphate-modified starch in a ratio by weight of 90/10 to 10/90. When these three are combined, it is preferable to use the oxidized starch and urea phosphate-modified starch in a ratio of 10/90 to 90/10 by weight.
For the modified polyvinyl alcohol, acetoacetyl -modified polyvinyl alcohol, diacetone-modified polyvinyl alcohol, silicon-modified polyvinyl alcohol, and amide-modified polyvinyl alcohol are preferably used. In addition to these, sulfo-modified polyvinyl alcohol and carboxyl-modified polyvinyl alcohol can be used. When a crosslinking agent is combined to react with these polyvinyl alcohols, more preferable results are obtained.
The amount of the water-soluble polymer added is preferably 10 and 90% by weight, and more preferably 30 to 70% by weight, with respect to the solid content of the coating liquid for the protective layer.
A moisture resistance enhancer (crosslinking agent) can be added to crosslink the binder component in the protective layer and further improve the storage stability of the heat-sensitive recording material. Examples of the moisture resistance enhancer include: N-methylolurea, N-methylolmelamine, water-soluble precondensates such as urea-formalin; methylolated phenol; polyamine compounds such as ethylenediamine; polyaldehydes including dialdehyde compounds such as glyoxal, and glutaraldehyde; dihydrazide compounds such as adipic acid dihydrazide, and phthalic acid dihydrazide; polyfunctional epoxy compounds; polyvalent metal salts (with Al, Ti, Zr, Mg, etc.), inorganic crosslinking agents such as boric acid, borax, and colloidal silica; and polyamide-epichlorohydrin.
When the polyvinyl alcohol and/or modified polyvinyl alcohol are used, the ratio of the crosslinking agent to these polyvinyl alcohols is preferably 2 to 30% by weight, and more preferably 5 to 20% by weight. By using the crosslinking agent, film strength and moisture resistance are improved. Polyaldehyde compounds and dihydrazide compounds are preferable as the crosslinking agent in the present invention.
By adding a surfactant to the coating liquid for the protective layer, better inkjet ink compatibility and chemical resistance are obtained. Preferable examples of the surfactant include salts of alkylbenzenesulfonates such as sodium dodecylbenzenesulfonates; salts of alkylsulfosuccinates such as sodium dioctylsulfosuccinate, as well as polyoxyethylene alkyl ether phosphates, sodium hexametaphosphate, and salts of perfluoroalkylcarboxylic acids. Among these, salts of alkylsulfosuccinates are more preferable. The amount of the surfactant is preferably 0.1 to 5% by weight, and more preferably 0.5 to 3% by weight, with respect to the solid content of the coating liquid for the protective layer.
The coating liquid for the protective layer may further include lubricants, defoaming agents, fluorescent brighteners and chromatic organic pigments as long as the effects of the prevent invention are not compromised. Examples of the lubricant include metal soaps, such as zinc stearate and calcium stearate, and waxes, such as paraffin wax, microcrystalline wax, carnauba wax and synthetic polymer wax.